The present invention relates to an improved process for producing aromatic carboxylic acids by catalytic liquid phase oxidation of a corresponding precursor in a suitable solvent. More particularly, the present invention is an improved process for catalytic liquid phase air oxidation of paraxylene to produce terephthalic acid which comprises sequential steps of feeding the reactants to a first reaction zone at elevated pressure wherein the temperature and the uptake of oxygen are controlled and any terephthalic acid which forms remains in solution, and then feeding the resulting reaction medium to a second reaction zone to complete the oxidation reaction.
Practically all terephthalic acid is produced on a commercial scale by catalytic, liquid phase air oxidation of paraxylene. Commercial processes use acetic acid as a solvent and a multivalent heavy metal or metals as catalyst. Cobalt and manganese are the most widely used heavy metal catalysts, and bromine is used as a renewable source of free radicals in the process.
Acetic acid solvent, air (molecular oxygen), paraxylene and catalyst are fed continuously into an oxidation reactor that is maintained at from 150° C. to 225° C. and from about 500 to 2,500 kPa (i.e., 5-25 atm). The feed solvent:paraxylene mass ratio is typically less than 5:1. Air is added in amounts in excess of the stoichiometric requirements for full conversion of the paraxylene to terephthalic acid, to minimize formation of undesirable by-products, such as color formers. The oxidation reaction is exothermic, and heat is removed by allowing the acetic acid solvent to vaporize. The corresponding vapor is condensed and most of the condensate is refluxed to the reactor, with some condensate being withdrawn to control reactor water concentration (two moles of water are formed per mole of paraxylene reacted). The residence time is typically 30 minutes to 2 hours, depending on the process. Depending on oxidation reactor operating conditions, e.g., temperature, catalyst concentration and residence time, significant degradation of the solvent and precursor can occur, which, in turn, can increase the cost of operating the process.
The effluent, i.e., reaction product, from the oxidation reactor is a slurry of crude terephthalic acid (TA) crystals which are recovered from the slurry by filtration, washed, dried and conveyed to storage. They are thereafter fed to a separate purification step or directly to a polymerization process. The main impurity in the crude TA is 4 carboxybenzaldehyde (4-CBA), which is incompletely oxidized paraxylene, although p-tolualdehyde and p-toluic acid can also be present along with undesirable color formers. By conducting the oxidation reaction according to the invention as described in greater detail below, it is possible to substantially reduce the formation of impurities in the final TA product and effectively control solvent and precursor degradation.